This invention relates to a composite material containing carbon and a matrix of silicon carbide and silicon and reinforced with carbon fibers, which material has relatively high elongation at break, and to a process for the production of such a composite material.
Composite materials having a high temperature resistant ceramic matrix reinforced with high temperature resistant fibers have met with remarkable success in many areas of advanced technology. They are used, for example, as a facing material for critical points on the outer skin of reusable space vehicles, for jet engine nozzle liners, for turbine blades, for highly stressed structural components in mechanical engineering and for friction linings. However, other than specific applications and use for development and test purposes, these materials have not hitherto been used more widely. The reason for this resides in their failure behavior. If defects, for example notches, are present in the surface or if defects occur or are present in the structure of articles consisting of these composite materials, they fail catastrophically with unrestrained crack propagation when loaded, as, unlike in metals, stress peaks cannot be dissipated by any sliding action in the crystal lattice. Such failure occurs, for example, in high strength silicon carbide reinforced with silicon carbide fibers which are perfectly bonded to the matrix. Since the occurrence of such failure is statistically highly variable, components made from these brittle composite materials frequently cannot fulfill the requirements placed upon them, especially if economic criteria must also be taken into account.
One objective of material development has thus been to reduce the brittleness of composite materials, i.e. to reduce their modulus of elasticity and to raise their elongation at break. One material which adequately exhibits this combination of properties is carbon reinforced with fibers of carbon or graphite (CFC). This material is thus used, for example, as a friction lining in high performance aircraft brakes. One disadvantage of this material is, however, the low oxidation resistance of the carbon, which results in high wear in CFC components if they cannot be kept under a protective gas. There are applications without inert gas protection such as brake material or as protective heat shields on high performance aerospace vehicles. While applying oxidation-inhibiting protective coatings has brought about some improvements, it cannot solve the problem completely.
One branch of material development then pursued the production of silicon carbide articles reinforced with fibers of carbon or graphite in which the C fibers are, on the one hand, protected from oxidation by the surrounding SiC matrix and, on the other, bonding of the C fibers is imperfect such that, while the fibers still provide a good reinforcing action, crack propagation is inhibited at the fiber interfaces by energy consumption and relatively elastic failure behavior is achieved. The production of such a material has hitherto been considered problematic because, at elevated temperatures, silicon and carbon very readily react to yield silicon carbide, i.e. the C fibers are at least partially converted into SiC, so losing their reinforcing action, and because production from C fibers and silicon carbide powder by, for example, hot pressing proved insufficiently successful.
One improvement to the situation was achieved by coating the carbon fibers using the CVD process (CVD=chemical vapor deposition) with protective layers of high-melting substances such as pyrocarbon, TiC, TiN or SiC before impregnation with liquid silicon (E. Fitzer et al. Chemie-Ingenieur-Technik 57, no. 9, pp. 737-746 (1985), in this case p. 738, right hand column). This protective action of pyrolytically deposited carbon is also exploited in DE-PS 39 33 039 C2, in accordance with which moldings made from short carbon fibers or carbon felts are initially coated with a first layer of pyrolytic carbon, then graphitized and subsequently provided with a second layer of pyrocarbon, before they are subjected to siliconization with liquid silicon. One disadvantage of this process is the use of the comparatively costly CVD or CVI process (CVI=Chemical Vapor Infiltration), in accordance with which, if it is used only once per process stage, microcracks remain in the pyrocarbon layers, into which silicon may subsequently penetrate and at least a proportion of the C fibers may be converted into SiC (c.f. loc. cit. Fitzer et al., p. 740, column 2) or, if multi-layer coatings are provided, production of the components is very costly.
According to another process (DE 44 38 456 Al), the process starts from specially arranged layers of bundles of continuous carbon fibers which are enclosed in a synthetic resin matrix. Once this synthetic resin article reinforced with carbon fiber bundles has been carbonized, the article exhibits, thanks to the particular production method used, substantially translaminar channels which are filled with liquid silicon during siliconization. The introduced silicon is then substantially reacted with the carbon matrix to yield silicon carbide (c.f. also DLR notice PR 10/89 A: WB-BK 4./1). In this case too, the production of the basic structure from long fiber non-woven fabrics is relatively elaborate, depending upon the arrangement of the C fiber bundles, the article exhibits properties which are anisotropic overall or for each ply and if one of the layers providing protection against oxidation degrades, the underlying layer of carbon fibers may always be exposed without protection.